Off-cut Angle Research Articles

(Invited) GaN Homoepitaxial Growth and Substrate-Dependent Effects for Vertical Power Devices

Vertical GaN power switch technology is expected to be utilized in next-generation medium to high voltage power converters due to the low ON-resistance and high breakdown voltage enabled by the improved critical electric field and mobility compared to Si and SiC-based devices. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth, allowing recent realization of several fundamental vertical power devices, including diodes with edge termination, trench MOSFETs, and CAVETs. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow vertical GaN power devices to achieve their full potential, notably the realization of repeatable thick drift layers with low background doping. To enable this, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for MOCVD growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. We have characterized multiple substrates from over eight suppliers and the homoepitaxy grown on those substrates. All the substrates had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling to detection of different concentrations of grain boundaries, sample impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conducive regions. The substrates can be characterized in two different categories: those with uniform characteristics, including carrier concentration, and those without. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Photoluminescence of the homoepitaxial surface along with Raman spectroscopy show that non-uniformities in the substrate carrier concentration can continue into the epitaxy. Following optical characterization, vertical Schottky diodes were fabricated to quickly evaluate device performance. While most of the films showed the ability to withstand high electric fields, those with more uniform properties in the substrate also showed more uniform electrical properties. These results show that the bulk substrates enable the path to high voltage vertical devices, but also show the significant influence that substrates can play in device performance.

Crystalline quality improvement of face-to-face annealed MOVPE-grown AlN on vicinal sapphire substrate with sputtered nucleation layer

Face-to-face annealed metalorganic vapor phase epitaxy (MOVPE)- grown AlN templates with sputtered AlN nucleation layers (FFA MOSp-AlN templates) were fabricated on vicinal sapphire substrates. The sputtered thin AlN layer supplies AlN seeds that are well aligned to the c-axis. The MOVPE growth with a relatively thick AlN enlarges crystal grains with a low concentration of impurities. Face-to-face annealing at high temperatures reduces the twist component of AlN films due to recrystallization, and AlN films with low threading dislocation densities are formed. The full width at half maximum (FWHM) of the (10−12) X-ray rocking curves (XRCs) of AlN templates decreased as the thickness of the MOVPE-grown AlN layer increased. The FWHMs for the (0002) and (10−12) XRCs of the 300-nm-thick AlN templates with an off-cut angle of 0.6° were 9 and 337 arcsec, respectively. The concentrations of O, Si, and C impurities in the FFA MOSp-AlN templates were lower than those in the face-to-face annealed sputtered AlN templates. Si doped Al0.7Ga0.3N films, which are typically used as n-type AlGaN layers for UVC LEDs, were grown on the FFA MOSp-AlN templates with various off-cut angles. The AlGaN films with smooth flat surfaces were obtained using the FFA MOSp-AlN template with larger off-cut angles due to the suppression of the large hillock formation.

Enhanced electromechanical coupling in SAW resonators based on sputtered non-polar Al0.77Sc0.23N 112¯ thin films

Non-polar a-plane Al0.77Sc0.23N 112¯0 thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3(11¯02) substrates. Different substrate off-cut angles were compared, and the off-cut angle of 3° resulted in the best structural quality of the AlScN layer. Structural characterization by x-ray diffraction confirmed that single phase, wurtzite-type, a-plane AlScN 112¯0, surface acoustic wave resonators were fabricated with wavelengths λ = 2–10 μm (central frequency up to 1.7 GHz) with two orthogonal in-plane propagation directions. A strong dependence of electromechanical coupling on the in-plane orientation was observed. Compared to conventional c-plane AlScN based resonators, an increase of 185–1000% in the effective electromechanical coupling was achieved with only a fractional decrease of <10.5% in series resonance frequency.

Low Threshold Quantum Dot Lasers Directly Grown on Unpatterned Quasi-Nominal (001) Si

We report electrically pumped, continuous-wave (cw) InAs/GaAs quantum dot (QD) lasers directly grown on quasi-nominal Si (001) substrates with offcut angle as small as 0.4°. No GaP, Ge buffer layers or substrate patterning is required. An anti-phase boundary free epitaxial GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) with a low threading dislocation density of 3 × 107 cm−2. Room-temperature cw lasing at ∼1.3 μm has been achieved, with a minimum threshold current density of 34.6 A/cm2 per layer, a maximum operating temperature of 80 °C, and a maximum single facet output power of 52 mW. A comparison of various monolithic III-V hetero-epitaxy on Si solutions is presented. Direct growth on unpatterned quasi-nominal (001) Si may yield the best material quality at the lowest lifecycle cost.

Growth and characterization of Tb silicide nanostructures on Si(hhk) substrates

The growth of Tb-induced nanostructures on vicinal Si(111) substrates with an offcut toward the $\left[11\overline{2}\right]$ direction, so-called Si(hhk) substrates, was studied. The nanostructures were analyzed using scanning tunneling microscopy and spectroscopy as well as core-level photoemission spectroscopy. For low Tb coverages, terraces covered by a Tb-induced submonolayer structure are observed. At higher coverages, metallic Tb disilicide monolayer structures are dominant. Their appearance strongly depends on the offcut direction. Well-defined nanowires with straight edges are only present for Si(hhk) substrates with $hlk$. Furthermore, only these substrates may host very narrow metallic nanolines. In general, narrower nanostructures are observed for larger offcut angles enabling the formation of Tb disilicide nanowires with average widths as low as $2.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for Si(335) substrates.

AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy

We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 109 cm−2. However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86%).

Thermodynamic model for metalorganic vapor-phase epitaxy of N-polar group-III nitrides in step-flow growth mode: Hydrogen, competitive adsorption, and configuration entropy

A thermodynamic model for metalorganic vapor-phase epitaxy (MOVPE) of the N-polar $(000\overline{1})$ binary group-III nitrides (AlN, GaN, and InN) in the step-flow growth mode is proposed based on the Burton, Cabrera, and Frank (BCF) theory. The coverages of the group-III adatoms are thermodynamically evaluated under competitive adsorption with hydrogen, which is used as a carrier gas or dissociated from the ${\mathrm{NH}}_{3}$ source gas during MOVPE. The chemical potentials of the group-III and H adatoms on N-polar group-III nitride surfaces are modeled using the respective bond energies with the surface N atoms of the nitride and the vibrational frequencies of the adatoms. The coverages of the coadsorbed group-III and H adatoms are calculated using the Langmuir adsorption isotherm with these chemical potentials. The configuration entropy of the group-III adatoms bridges the gap between the thermodynamic model and the BCF theory. The coverage of the group-III adatoms plays a role like partial pressure of the group-III gas in the thermodynamics. The equilibrium coverage of the group-III adatoms and the equilibrium pressure of the ${\mathrm{NH}}_{3}$ gas are evaluated from the conditions of Gibbs energy balance between the sources (group-III adatom and ${\mathrm{NH}}_{3}$ gas molecule) and products (group-III nitride and $3/2\phantom{\rule{0.28em}{0ex}}{\mathrm{H}}_{2}$ gas molecules) and of speed balance between group-III and N incorporation into step kinks. Fair agreements with the experimentally optimized growth conditions for MOVPE of N-polar GaN and InN are obtained by this method. Among the examined binary group-III nitrides, AlN growth is hardly affected by ${\mathrm{H}}_{2}$ gas pressure, GaN growth is controlled well by ${\mathrm{H}}_{2}$ gas pressure, and InN growth is strongly inhibited by ${\mathrm{H}}_{2}$ gas. A criterion for selecting the ${\mathrm{NH}}_{3}$/group-III flow ratio for maximum products/cost and minimum waste of the materials is demonstrated using the growth model and the estimated growth parameters. The offcut angle dependence of the growth rate on the vicinal substrates is also investigated.

Influence of substrate off-cut angle on the performance of 310 nm light emitting diodes

The influence of the sapphire substrate off-cut angle from c-plane orientation on the surface morphology of AlN/AlGaN layers and the performance of 310 nm light emitting diode structures has been investigated. Increasing the off-cut angle from 0.1° to 10° towards the m-plane leads to a change of the (Al,Ga)N growth mode, threading dislocation density, and emission characteristics of ultraviolet light emitting diodes (LEDs). The growth mode changed from step-flow with bilayer steps to step-bunching and finally to rough 3D growth. The lowest threading dislocation density has been observed for structures deposited on sapphire with an off-cut angle of 0.65°, whereas the emission power is the highest for structures grown on substrates with an off-cut of 0.48°. Electroluminescence measurements display single peaks for structures on substrates with off-cut angles of up to 0.65°. The emission peak full width at half maximum remains nearly constant (~9.5 nm) for structures with off-cut angels up to 0.21° and increases to 11.7 nm and 21.0 nm for 0.48° and 0.65°, respectively. The emission wavelength is almost unchanged by the use of off-cut angles up to 0.48° (311 nm) and increases to 319 nm for 0.65°. Larger off-cut angles cause multiple emission peaks indicating compositional inhomogeneity of the InAlGaN quantum wells.

(Invited) Homoepitaxial GaN Growth and Substrate-Dependent Effects

Vertical GaN power switch technology is expected to be utilized in next-generation medium to high voltage power converters due to the low ON-resistance and high breakdown voltage enabled by the improved critical electric field and mobility compared to Si and SiC-based devices. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth, allowing recent realization of several fundamental vertical power devices, including diodes with edge termination, trench MOSFETs, and CAVETs. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow vertical GaN power devices to achieve their full potential, notably the realization of repeatable thick drift layers with low background doping. To enable this, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for MOCVD growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. Four different substrates - three grown by hydride vapor phase epitaxy and the fourth by ammonothermal technique were obtained from multiple vendors. All had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling to detection of different concentrations of grain boundaries, sample impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conducive regions. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Donor incorporation in the epitaxy is relatively uniform, regardless of the substrate doping. However, homoepitaxy on substrates with inhomogeneous impurity incorporation did show some near band edge broadening by PL. Carbon incorporation varied with substrate offcut angle, which is supported by DFT calculations. All interfaces between epitaxy and substrate have a large Si peak, which requires further study. Following optical characterization, vertical Schottky diodes were fabricated to quickly evaluate device performance. While most of the films showed the ability to withstand high electric fields, those with more uniform properties in the substrate also showed more uniform electrical properties. These results show that the bulk substrates enable the path to high voltage vertical devices, but also show the significant influence that substrates can play in device performance.

Large thermopower anisotropy in PdCoO2 thin films

The delafossite compound PdCoO${}_{2}$ is composed of highly conducting Pd and insulating CoO${}_{2}$ layers. As a consequence of this lattice architecture, the thermopower of PdCoO${}_{2}$ was predicted to be extremely anisotropic. Because of the limited size of available single crystals, however, these predictions had not been tested experimentally. The authors of this paper show that the electric and thermoelectric transport parameters of PdCoO${}_{2}$ along the main crystallographic directions can be determined from measurements on thin films grown on substrates with different offcut angles. The method is applicable to a wide range of thermoelectric materials. The experimental results confirm the predicted thermopower anisotropy of PdCoO${}_{2}$ and thus provide interesting perspectives for thermoelectric device applications.

Growth temperature dependence of cubic GaN step structures and cubic InN dot arrays grown on MgO (001) vicinal substrates

In this paper, we report on the detailed structural features of step-bunched surface formed on cubic (c-) GaN and self-assembled c-InN dot arrays grown on the c-GaN surface, particularly focusing on the growth temperature dependence. Samples were fabricated on MgO (001) vicinal substrates with off-cut angles of 2.0° and 3.5° toward [110] by RF-MBE. Multisteps and terraces with a certain periodicity in the vicinal direction were observed on c-GaN layers. The average terrace width narrowed with lowering the growth temperature and increasing the substrate off-cut angle. The formation of c-InN dots proceeded in the Stranski–Krastanov growth mode with critical thickness of 0.54–0.66 nm. The c-InN dot exhibited a variety of structural features depending on the growth conditions. Positional dot alignment along the multistep edges of the underlayer was observed under some conditions. The degree of alignment was found to be affected by the terrace width on the c-GaN underlayer.

Dependence of thermal stability of GaN on substrate orientation and off-cut

The thermal stability of ±c- and m-plane GaN substrates and its dependence on the m-plane GaN off-cut angle were investigated at and above 1200 °C at atmospheric pressure in flowing N2 with and without added NH3. It was clarified that decomposition of the +c-plane GaN surface is promoted by the addition of NH3, while decomposition of the –c- and m-plane GaN surfaces is suppressed by added NH3. It was found that –c- and m-plane GaN substrates can be heated at 1300 °C with an added NH3 input partial pressure of more than 0.2 atm, which is 100 °C higher than for +c-plane GaN substrates. Nevertheless, decomposition of m-plane GaN substrates became noticeable with increase in the off-cut angle of the substrate. It was revealed that the decomposition was better controlled by using m-plane GaN substrates with an off-cut towards [0001̅] than towards [0001].

On the optimum off-cut angle for the growth on InP(111)B substrates by molecular beam epitaxy

InGaAs and InAlAs epilayers were grown on InP(111)B substrates by molecular beam epitaxy. Rather than focusing on a specific off-cut angle, the growths were done on rounded wafer edges, which expose a broad spectrum of vicinal surfaces with varying off-cut angle and off-cut azimuth. The epilayers were grown at several different growth conditions by varying the growth temperature, growth rate, and arsenic (As) overpressure. The epitaxial layers were characterized at the center and the edge of the wafers using Nomarski differential interference contrast microscopy and atomic force microscopy. It was shown that a minimum misorientation angle of ∼0.4° should be used in order to avoid pyramidal hillocks. At higher misorientations, 1.7°–3°, step bunching can lead to surface roughening.

Hillock-free and atomically smooth InSb QWs grown on GaAs substrates by MBE

Comprehensive studies on the surface morphological evolution of AlInSb metamorphic buffers and InSb QWs grown on top were conducted as a function of the GaAs (001) substrate offcut angles. We confirmed our earlier postulation that the vicinal surfaces defined by the hillock facets have the exact surface orientation needed to achieve large-area hillock-free surfaces. The related morphological transitions were discussed with a graphic illustration. The optimum substrate offcut for InSb towards [1¯10] direction was found to be around 0.5–0.6° with our growth conditions. On 2-inch GaAs (001) substrates with this offcut, a hillock-free and atomically smooth surface morphology was successfully achieved for modulation-doped InSb QWs.

Direct observation of long distance lateral transport in InGaN/GaN quantum wells

The horizontal excitation energy transport in the range of tens of micrometers was measured in high quality homoepitaxial InGaN quantum wells (QWs) with the use of time and space resolved micro-photoluminescence as a function of (i) applied vertical electric field, (ii) temperature, and (iii) linear density of atomic steps. The investigated structure consisted of InGaN QWs inside a p-n junction. The indium content in QWs was designed to be different in defined areas of the sample (due to mastering of different off-cuts and atomic steps density) so that the wells could emit at energies from 2.6 to 2.86 eV. The horizontal transport range was sensitive to the vertical electric field, which means that it could not be just a radiation transfer, but charge carriers must have been involved as well. We found that the transport range decreased for higher slope angles, possibly due to stronger scattering on atomic steps when their linear density became higher. The diffusion coefficients reached 6 cm2/s and due to long lifetime of even 2 μs, the diffusion length was even LD = 30 μm in areas of low off-cut angles. We discuss possible mechanisms of transport and conclude that for such high diffusion constant, the most probable is the excitonic transport. The LD was maximum at 40 K and then decreased significantly with temperature, which was probably caused by thermal dissociation of excitons.

Two-photon-excited, three-dimensional photoluminescence imaging and dislocation-line analysis of threading dislocations in 4H-SiC

This paper describes the three-dimensional imaging of threading edge dislocations (<a>TEDs), pure c threading screw dislocations (<c>TSDs), and c + a threading mixed dislocations (<c + a>TMDs) in 4H silicon carbide (4H-SiC) epilayers by two-photon-excited photoluminescence (2PPL), where inclinations of dislocations in epilayers with different offcut angles are investigated. A numerical model based on carrier diffusion explains the mechanism of the dark-contrast 2PPL imaging provided by band edge emission. More than 450 threading dislocations in 4H-SiC epilayers are visualized three-dimensionally and are classified into six <a>-type TEDs, two <c>-type TSDs, and twelve <c + a>-type TMDs according to the directions of extra half-planes and the chiral geometries (right- or left-handed screw). The 2PPL images reveal that all threading dislocation types incline in the step-flow direction, the extent of which is more prominent for a larger offcut angle. The <a>TEDs also incline in the directions of their extra half-planes. The inclinations of <c>TSDs perpendicular to the step-flow direction are influenced by chirality, while those of <c + a>TMDs are influenced by both the directions of the extra half-planes and the chirality. The origins of the dislocation inclinations are discussed taking into account the kinetic interactions between the dislocations and the advancing steps on the growing surface.

The Role of Misorientation in Direct Wafer Bonded III-V Materials

Direct wafer-bonding is a promising approach to materials integration for heterojunction structures not realizable through epitaxial means. Without lattice-parameter mismatch restrictions, wafer-bonding opens up a myriad of possibilities of materials combinations for many novel electronic device structures. In fact, wafer-bonding of III-V semiconductor materials was used to fabricate high-efficiency multijunction solar cells [1,2]. In this work, the effect of misorientation (both relative tilt and rotation) on the electronic properties across the bonded interface was studied by fabricating GaAs and InP homojunction and heterojunctions. To study the role of relative tilt, (001) n-type GaAs and (001) n-type InP wafers with offcut angles of 4° towards <111>A and nominal on-axis substrates were bonded to form relative tilts of 0°, 4°, and 8°. Current-voltage (I-V) curves were measured at various temperatures ranging from 90 K to 340 K to determine the interfacial barrier heights using the Seager and Pike electron model [3]. Then, to study the effect of rotation, (001) n-type InP wafers with twist angles ranging from 0° to 90° in 15° increments were bonded. Prior to bonding, the wafers were first cleaved into quarters and submerged in an NH4OH solution to remove the native surface oxide and then submerged in a 20% aqueous (NH4)2S solution for 5 minutes. After drying the wafers with N2, the wafers were brought face-to-face and very low pressure (kPa) was applied manually at room temperature and in ambient air pressure. The relative rotational misalignment was induced by rotating the top wafer prior to applying pressure. The wafers were then annealed at 400 °C for 2 hours to strengthen the bonded interface. Metal Ohmic contacts were deposited on each semiconductor surface and samples were diced into ~2 x 2 mm squares. We find that higher relative tilt across the interface leads to larger barrier heights. For example, in the GaAs/GaAs system the barrier height increased from 0.56 eV to 1.00 eV when increasing the tilt from 0° to 8°, respectively. Additionally, systems with at least one side InP (whether in an InP homojunction or InP/GaAs heterojunction) corresponded to lower barrier heights than the GaAs homojunction system. Regarding rotation, there is a discrepancy in literature on the effect of rotational misalignment. Kish et al. [4] claimed rotationally misaligning (001) III-V wafers by 20° and 90° result in the same decrease in electrical conductivity across the bonded interface compared to their 0° bonded wafers. However, Okuno et al. [5, 6] found that 0° and 90° bonded samples result in nearly equal electrical performance. Our findings show that the lowest interfacial resistance corresponded to misorientations of 0° and 90° while the greatest resistance corresponded to 45°, which was 30% higher than the lowest interfacial resistance. We conclude that the effect of rotational misalignment on the electrical conduction across bonded interfaces follows a sinusoidal pattern and its periodic effect on interfacial conductivity depends on the lattice symmetry of the wafer’s surface orientation, which has implications for bonding wafers with different surface symmetry. Electrical characterization of the interface enables us to understand the key factors that influence the properties of a bonded interface. We concluded that surface passivation, materials choice, and relative misorientation of the bonded materials are the determining factors that control the electrical performance of the bonded structure.

Influence of template properties and quantum well number on stimulated emission from Al0.7Ga0.3N/Al0.8Ga0.2N quantum wells

AlGaN multiple quantum well laser heterostructures for emission around 240 nm have been grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown (ELO) AlN/sapphire templates. The edge emitting laser structures showed optically pumped lasing with threshold power densities in the range of 2 MW cm−2. The offcut angle of the sapphire substrates as well as the number and the width of the quantum wells were varied while keeping the total thickness of the gain region constant. A larger offcut angle of 0.2° leads to step bunching on the surface as well as Ga accumulation at the steps, but also to an increased inclination of threading dislocations and coalescence boundaries resulting in a reduced dislocation density and thus a reduced laser threshold in comparison to lasers grown on ELO with an offcut of 0.1°. For low losses, samples with fewer QWs exhibited a lower lasing threshold due to a reduced transparency pump power density while for high losses, caused by a higher threading dislocation density, the quadruple quantum well was favorable due to its higher maximum gain.

N-polar GaN/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor formed on sapphire substrate with minimal step bunching

The metal–insulator–semiconductor (MIS) gate N-polar GaN/AlGaN/GaN high-electron-mobility transistor (HEMT) on a (0001) sapphire substrate, which can be expected to operate with lower on-resistance and more easily work on the pinch-off operation than an N-polar AlGaN/GaN HEMT, was fabricated. For suppressing the step bunching and hillocks peculiar in the N-polar growth, a sapphire substrate with an off-cut angle as small as 0.8° was introduced and an N-polar GaN/AlGaN/GaN HEMT without the step bunching was firstly obtained by optimizing the growth conditions. The previously reported anisotropy of transconductance related to the step was eliminated. The pinch-off operation was also realized. These results indicate that this device is promising.

GaN-Based Blue Laser Diodes With 2.2 W of Light Output Power Under Continuous-Wave Operation

Research on GaN-based blue and green laser diodes (LDs) has attracted great attention in the past years to meet the demand for laser display application. In this letter, we improve luminescence homogeneity and reduce internal loss of GaN-based blue LD structures grown on $c$ -plane free-standing GaN substrates. The morphologies of homoepitaxial GaN layers are greatly affected by offcut orientation and an angle of GaN substrates. Perfect morphology with straight atomic steps and regular terrace width can be obtained for GaN layers grown on GaN substrates with an offcut angle of 0.42° toward $m$ -plane. On GaN substrates with an offcut angle of 0.4° toward $m$ -plane, homoepitaxial LD structures grown utilizing the same growth temperature for InGaN quantum wells and GaN quantum barriers have narrowest emission linewidth and very homogeneous luminescence. The internal loss is greatly reduced by optimizing Mg doping profile. As a result, we have achieved GaN-based blue laser diodes with 2.2 W of light output power under continuous-wave operation at room temperature.